This invention relates to electric proximity switches and more particularly, it concerns liquid level responsive switches of a type especially suited for use with submersible sump and sewage pumps.
A major problem with which the submersible sump and sewage pump industry has and continues to face is the need for a reliable liquid level responsive switch capable of withstanding repeated maintenance-free operation for long periods of time in the environment in which such pumps are used. For example, the humid atmosphere to which a switch of this type is continuously exposed necessitates either the use of highly exotic and expensive materials in operative switch components to avoid malfunction as a result of corrosion or a complete isolation of such components from the atmosphere. While the switch itself is not intended to operate while submerged, the possibility of submergence as a result of flood conditions, for example, must be accounted for in the switch design in order to avoid the electrical shock hazards incident to submersion.
Because of the predictable gravitational orientation of liquid level responsive switches, float operated mercury switches have been looked upon as a likely candidate for a solution principally because of the facility provided for hermetically encapsulating the switch terminals or electrodes. In particular, switches in which a tiltable mercury capsule is moved between open and closed positions by a float or buoyant weight carried permanent magnet have shown promise primarily because of the capability for isolating all of the electric and mechanical switch components in a sealed enclosure, leaving only the actuating magnet exposed to the humid atmosphere or to be wetted on submersion of the switch. Although magnetically actuated mercury switches of this type have approached solutions of the basic problems associated with submersible sump and sewage pump switches, there is need for improvement in a number of respects. For example, tilting movement of the hermitically sealed mercury capsule and switch electrodes requires a measure of flexibility in the leads extending to the capsule such that the current carrying capacity of the leads and thus of the switch is limited. Although the need for lead flexibility may be circumvented by force multiplying mechanical linkages capable of transmitting movement of the float carried magnet to capsule tilting movement, such linkages add both to manufacturing costs and to the potential for malfunction. In addition, tiltable mercury switches are extremely sensitive to gravitational orientation and therefore require a degree of care in installation difficult to achieve in many submersible pump environments. Also in such switches, there is potential for fouling the actuating magnet either mechanically such as by trash preventing its movement with the float to which it is attached or magnetically such as by permeable trash material tending to reduce the effective force of the magnet.